1. Field of the Invention
The invention relates to the petroleum industry and notably oil exploration through the study of sedimentary basins. The method of the invention can be used in any application requiring characterization of the geometry and of the nature of the layers making up a sedimentary basin and in particular (1) in oil exploration during the study of boundary zones, the evaluation of new fields and the characterization of oil reservoirs, and (2) in geological storage of gas and fluids such as CO2 or hydrogen. In particular, the invention is a method allowing determination of the distribution of fine and coarse sediments in a sedimentary basin, over large time and space scales (duration of the order of some thousand years to some hundreds of million years and horizontal distance of the order of several tens of to several hundred kilometers).
2. Description of the Prior Art
The major current trends in oil exploration are on the one hand moving towards increasingly complex zones (mountain range foothills, ultra-deep marine zones, shale gas, etc.) and, on the other hand, in optimizing recovery from already discovered reservoirs. Similarly, storage of more and more resources or waste in geological sites is desired. In order to meet these two objectives, it is important to best characterize the distribution of sediments in sedimentary basins and, in particular, the relative distribution of the coarse sediments and of the fine sediments. Indeed, these two types of sediment play a major role regarding the flow properties of fluids in rocks. The coarse sediments are generally permeable and the fine sediments are impermeable. Good characterization of a sedimentary basin thus requires good understanding of the stratigraphy of the basin, that is the geometry of the sedimentary layers and the distribution of the fine and coarse sediments within these layers.
Tools and notably computer tools (computer, software, etc.) are used to carry out simulations allowing obtaining a representation (referred to as stratigraphic simulation) that describes the stratigraphic architecture of a sedimentary basin. To achieve these simulations, a tool is used, known as a stratigraphic (numerical) model, allowing accounting for these aspects in an approximate manner. A stratigraphic model is a set of equations simulating the evolution over time of the sedimentary basin, as a result of three major processes which are (1) the creation (or elimination) of a space available for sedimentation by tectonic, eustatic and flexural movements, (2) the sediment supply to the basin, either through boundaries, or through the agency of in-situ production or precipitation, and (3) the transport of these sediments in the available space created.
Such a tool (stratigraphic model) is used in software for simulating deterministically the stratigraphic response of the sedimentary system to the variations over time of physical processes. The DionisosFlow® software (IFP Energies nouvelles, France) is an example of such a software.
Two main approaches are conventionally used which are (1) geometric and geostatistical approaches, wherein the geometry of the horizons and the nature of the rocks are defined manually or mathematically, and (2) deterministic approaches based on a simulation of the sedimentary processes which are basin deformation, sediment supply and transport of these sediments to the basin. The present invention contributes to the second approach, generally referred to as “stratigraphic forward modelling.”
This type of stratigraphic modelling appeared in the 1970s, in particular with the work carried out by Harbaugh and Bonham-Carter, 1970, who provided a series of very simple models. This precursory work (Tetzlaff and Harbaugh, 1989, Martinez and Harbaugh, 1993) gave rise to many stratigraphic models, generally classified in two major families which are process-based stratigraphic forward models and slope-based stratigraphic forward models.
Process-based models relate to the evolution of a sedimentary system over short periods of time, generally of the order of years to thousands of years. The first one is the Sedsim model (e.g. Tetzlaff and Harbaugh, 1989, Martinez and Harbaugh, 1993, Li et al., 2006, Salles et al., 2010), which simulates the evolution of shallow clastic and carbonate environments, and of turbiditic environments in deep marine domains. This model allows determining the stratigraphic architecture of sedimentary reservoirs and formations over relatively long periods of time (years to hundreds of thousands of years). More recently, many research teams became interested in process-based stratigraphic modelling (Driscoll and Karner, 1999, Harris and Wiberg, 2001, Bitzer and Salas, 2002, Kubo et al., 2005, Overeem et al., 2005). These models are concerned with the evolution of a sedimentary system over short periods of time (some tens of thousands of years being the maximum). These models thus rely on an extensive description of physical processes, in particular water flow in the fluvial domain or the marine domain. They can thus simulate in detail transient phenomena such as the flow of a dense current on the sea bottom, or the migration of sand dunes under the action of a beach drift or of a marine current. However, they generally focus on a single depositional environment which is the continental and fluvial domain, the deltaic domain, or the deep marine domain, having either clastic or carbonate sediments.
Slope-based models use a simpler description of the physical processes and assume that the sedimentary system is in dynamic equilibrium. That is the water flow is stationary and the sediment transport can be described by a diffusion equation relating the transported sediment flux to the slope of the basin (hence the name of this model family). The first diffusive models were developed in order to study in 2 dimensions (x,z) the evolution of mountain ranges, as well as the evolution of scarps, deltas and clastic margins. The most recent work has allowed showing that using a 3D diffusion equation enables correct description of the evolution of sedimentary systems having coarse sediments (sand, gravel) such as alluvial cones, deltas and deep marine turbiditic systems, as well as carbonate systems and mixed systems (clastic and carbonate). EP Patent 1,435,527 for example describes a modelling method for generating a 2D or 3D diffusive type stratigraphic model allowing simulation of the multilithologic filling of a sedimentary basin over geological time periods. According to this method, modelling is based on the numerical simulation of the evolution of a sedimentary basin, from past to present, in a series of time intervals. At each time interval, three major phenomena interact and are modelled numerically which are the basin deformation, the sediment supply and the transport of these sediments in the deformed basin. The long-term permanent transport (reptation process, slow diffusion process, etc.), the short-term transport (induced by rains and floods) and the catastrophic transports (due to cliff collapses notably) are taken into account for sediment transport modelling, using an exponential water velocity model.
The main advantage of slope-based models is to allow fast and realistic modelling of the evolution of sedimentary systems over long time periods and large distances. These models are thus increasingly used in the petroleum field to evaluate exploration scenarios or to better characterize petroleum systems, and various approaches have been patented recently, but they are dedicated to coarse sediment modelling and do not take account of fine sediments.
These various methods involve certain drawbacks:                process-based stratigraphic models do not allow studying the evolution of sedimentary systems over geologic times (millions to hundreds of millions of years), due to the fineness, as well as the complexity of the description of the physical processes that are involved. Furthermore, most models focus on short-term processes (thousands of years being the maximum) and therefore disregard the impact of subsidence or compaction on the stratigraphy of sedimentary basins,        slope-based stratigraphic models are ideal for the time and space scales considered in the present invention, but they are dedicated to coarse or mixed carbonate or clastic systems, thus disregarding suspended fine sediments. Using a diffusion equation of the topography allows simulation of the evolution of a topography over long time periods and obtaining a quite realistic description of the transport of coarse sediments. However, these models are ill-suited for simulation of the transport of fine particles that move mainly through advection and diffusion in the marine domain.        